In radio frequency transceivers, it is necessary to provide a switching function which alternately connects the transmitter or receiver to the antenna. During transmission, the transmitter is connected to the antenna, and the receiver is disconnected from the antenna. During reception, the receiver is connected to the antenna, and the transmitter is disconnected from the antenna. If the transmitter and receiver were connected simultaneously to the antenna, the relatively large power output from the transmitter could damage the receiver.
An ideal switch for this purpose should introduce minimal loss in the transmit or receive channel when it is turned on (e.g., less than 1 dB). The switch should also have good isolation characteristics, which is to say that the leakage through the switch should be minimal when the switch is turned off. Finally, the switch should consume minimal power and should possess a good input/output power handling capability, i.e., the input power versus output power should be as linear as possible.
A simple MOSFET switch, as illustrated in FIG. 1, may be used for this purpose. RF.sub.in represents the incoming radio frequency signal and RF.sub.out represents the outgoing radio frequency signal. The problem with this configuration is that, particularly in wireless applications, the available supply voltage is limited. Referring to FIG. 1, this means that V.sub.control could be limited to, for example, 3 V. As RF.sub.in approaches V.sub.control, the drain current and thus the power handling capabilities of MOSFET M1 are restricted.
One possible solution is to add a parallel P-channel MOSFET, as shown in FIG. 2. In the arrangement shown in FIG. 2, a P-channel MOSFET P1, whose gate is driven through an inverter 20, essentially takes over most of the current and power handling requirements of the switch when MOSFET M1 is operating with a limited gate-to-source voltage. Conversely, when MOSFET P1 is restricted, MOSFET M1 absorbs most of the power handling requirements.
A possible problem with the arrangement of FIG. 2 is that the combined parasitic capacitance of MOSFETs M1 and P1 may become excessive, leading to large losses at high frequencies. This is particularly true because P-channel MOSFET P1 must normally be larger than N-channel MOSFET M1 in order to obtain an equivalent on resistance, and this further increases the parasitic capacitance of the device. In general, the parasitic capacitance of the switch illustrated in FIG. 2 might be on the order of three times that of the switch illustrated in FIG. 1.
Accordingly, what is required is a radio frequency switch which has an acceptable level of parasitic capacitance while providing good power handling capabilities as RF.sub.in approaches V.sub.control.